General processes by which geothermal brine can be used to generate electric power have, of course, been known for some time. Geothermal brine from a producing well, having a wellhead temperature of over about 180.degree. C. and a wellhead pressure of over about 400 psig, for example, can be flashed to a reduced pressure to convert some of the water or brine into steam. Steam produced in this manner is generally used in conventional steam turbine-type power generators to generate electricity.
Cooler, less pressurized, geothermal brine can be used in closed-loop, binary fluid system in which a low-boiling point, secondary liquid (such as hydrocarbon) is vaporized by the hot brine. The vapor produced from the secondary liquid is then used in a gas turbine-generator to generate electricity. The vapor from the turbine is recondensed and reused.
In both instances, the "used" geothermal brine liquid is most commonly reinjected into the ground into a "reinjection well" to replenish the aquifer from which it was extracted and to prevent ground subsidence. Reinjection of geothermal brine into the reinjection well is also important to avoid the problems associated with the disposal of large amounts of saline and/or highly-contaminated and nearly saturated brine.
It is generally known that the solubility of most dissolved ions in geothermal brine decreases with a decrease in brine temperature. If dissolved ions are present near their saturation concentration in the brine, a significant reduction in the temperature of the system can result in supersaturation and precipitation of a portion of these supersaturated ions. Precipitates can combine and deposit as a scale on any solid surface with which they come into contact, such as the vessel or conduit in which the brine is confined.
As discussed by Bowen and Groh ("Energy Technology Handbook," D. M. Considine, Editor, at page 7-4 of Chapter 7 entitled "Geothermal Energy"), liquid-dominated geothermal brine reservoirs may be conveniently divided into two types: one type having high-enthalpy fluids above 200 calories/gram; and one having low-enthalpy fluids below this point. High temperature type brines (i.e., high-enthalpy brines) have been defined by in-situ reservoir temperatures, the high temperature type having in-situ temperatures generally above 180.degree. C., typically above 200.degree. C., most commonly above 220.degree. C., whereas the low temperature type (i.e., low-enthalpy brines) have temperatures below these values. The high-enthalpy brines especially tend to dissolve reservoir rock or contacting solids and these brine types contain dissolved solids (and ions) in concentrations ranging from around 2,000 to as much as 260,000 ppm by weight.
An especially troublesome dissolved solid component of the high-enthalpy brine is silicon, which may be found at or near saturation concentrations in the form of oligomers or polymers of silicic acid. Such species tend to precipitate out of the brine at almost every stage of brine processing as the temperature is lowered, for example, as substantially pure silica, as a tightly adherent metal-silica/metal-silicate scale, or as other solidified silicon-containing components. Unless inhibited, naturally occurring silica-rich scale/precipitation (as the brine is cooled) must be removed frequently. The precipitation tendency (and the consequent need to remove precipitate) increases as lower brine temperatures are reached during the cooling process.
During the extraction of the thermal energy from a liquid geothermal brine, the brine temperature is reduced. Heat-exchangers are commonly used for low-enthalpy brine applications, such as those for producing hot water. The brine's thermal energy is transferred within the heat-exchangers to the hot water. The heated water may in turn heat air (for space heating) or other fluids such as hydrocarbons (in a binary fluid system). Even though the low enthalpy brines may be saturated with dissolved solids, the limited amount of temperature reduction possible for these low-enthalpy (i.e., moderate temperature) brines produces little or no precipitation and fouling of heat-exchange surfaces, or plugging of injection wells. This lack of significant precipitation or fouling is also believed due to the relative stability of slightly supersaturated brines. Even if the supersaturated brine is not stable, the low precipitation rates (i.e., slow precipitation kinetics) at the moderate brine temperatures within these heat-exchangers are also believed to inhibit large amounts of precipitation and fouling.
However, high-enthalpy or high temperature brines typically have larger saturation concentrations of dissolved solids and faster precipitation kinetics. The removal of larger amounts of heat can also produce significant levels of supersaturation. High-enthalpy brines therefore tend to produce copious quantities of scale which can plug conduits, injection wells, the subterranean formation in the vicinity of the immediate injection wells (up to about 50 feet from the wellbore), and quickly foul a conventional heat-exchanger. Normally, conventional heat-exchangers are not generally employed for high-enthalpy brines, even though extraction of heat from such brines using a heat-exchanger process may otherwise be beneficial.
Because of such conventional heat-exchanger fouling, a condensing flash-method for extracting energy from high-enthalpy brines is commonly used upstream of the heat-exchanger. Flashing is accomplished in a vessel where brine pressure is reduced. As a result, a portion of the brine is flashed to steam and other gases while the temperature of the residual brine is decreased and separated from the steam. Flashing is often accompanied by massive amounts of precipitation formation that may scale and eventually plug piping. Other processes which avoid a fouled heat transfer surface, such as total flow and direct-contact (fluid-to-fluid) heat-exchange processes, have also been proposed for high-enthalpy brines.
Because of massive scaling, various proposals have been made to decrease the scale formation in flash-condensing or other non-heat-exchange surface equipment used in producing and handling high-enthalpy geothermal brines. In "Field Evaluation of Scale Control Methods: Acidification," by J. Z. Grens et al, Lawrence Livermore Laboratory, Geothermal Resources Council, Transactions, Vol. 1, May 1977, there is described an investigation of the scaling of turbine components wherein a geothermal brine at a pressure of 220 to 320 p.s.i.g. and a temperature of 200.degree. to 230.degree. C. (392.degree. to 446.degree. F.) was expanded through nozzles and impinged against static wearblades to a pressure of 1 atmosphere and a temperature of 102.degree. C. (215.degree. F.). In the nozzles, the primary scale was heavy metal sulfides, such as lead sulfide, copper-iron sulfide, zinc sulfide and cuprous sulfide. Thin basal layers of fine-grained, iron-rich amorphous silica appeared to promote the adherence of the primary scale to the metal substrate. By contrast, the scale formed on the wearblades was cuprous sulfide, native silver and lead sulfide in an iron-rich amorphous silica matrix. When the brine which originally had a pH of 5.4 to 5.8 was acidified with sufficient hydrochloric acid to reduce the pH of the expanded brine to values between 1.5 to 5.0, scaling was dramatically reduced or eliminated.
However, such acidification, especially at a pH near 1.5, tends to significantly increase the corrosion of the brine-handling equipment. If a downstream heat-exchanger were to be used to handle strongly acidified brines, added wall thickness or excessively costly materials of construction would be required. If added wall thickness heat-exchangers are used, frequent removal of corrosion products from the heat-exchange surfaces may also be required.
Strong acid treatments can also cause other geothermal fluid handling problems, such as the introduction of oxygen into an otherwise oxygen-free brine, the embrittlement of equipment, and the problems associated with reinjection into a subterranean formation. Common commercial acid treatments of geothermal brines have often been limited to relatively small changes in pH such as those treatments disclosed in my U.S. Pat. Nos. 4,500,434, and 5,190,664, the disclosures of which are incorporated by reference herein in their entireties. In U.S. Pat. No. 4,500,434, the moderately acidified brine was flashed in a series of separators and the formation of insoluble silicon components in the brine (and on the solid container surfaces) was substantially inhibited until disposal of the brine. In U.S. Pat. No. 5,190,664, a limited amount of sulfuric acid was added to a high-enthalpy brine prior to the brine passing through the mild steel heat-exchanger and silica scaling was virtually eliminated while corrosion rates were not significantly increased. These treatments accept a residual amount (not the complete elimination) of scale, especially silica, deposited on flash process or heat-exchange equipment in return for acceptable corrosion rates and significant reductions in scaling rates. Reducing scale formation decreases the amount of scale removal, but deposits can still quickly foul solid surfaces making such flash and/or heat-exchange processes impractical without very frequent cleaning--which may, in turn, result in partial or complete shut down of the process, i.e., shorten the process cycle life.
While the aforementioned acidified geothermal brine and modified acidified brine treatments have met with some success in some heat-exchanger and flash-separator (i.e., condensing-flash) surface applications, the need exists for a further improved treating process that further decreases fouling due to scaling by silicon-containing solids. Controlling fouling tendencies in materials commonly used in heat-exchangers or flash-separators, without significant added cost, would allow economic energy extraction from some high-enthalpy brines. The economic advantages of being able to extract energy in a condensing-flash process is beneficial when high-enthalpy brines contain precipitable components in near saturation amounts, but is especially beneficial in a heat-exchange process when the high-enthalpy brines contain high dissolved gas contents, thus avoiding the need for costly non-condensable gas removal equipment normally required for a condensing-flash process.
Accordingly, this invention provides an improved method for decreasing or essentially eliminating the overall precipitation and scaling of these brines, particularly silica and aluminum-silicate and/or iron-silicate scale, so as to prevent significant fouling of condensing-flash surfaces or heat-exchanger surfaces. It is also desirable to control corrosion of such surfaces when they are composed of commonly used materials of construction, such as low carbon steels.